With the Islamic warriors of ISIS having captured all the border posts between Iraq, Syria and Jordan, we may be witnessing the end of Sykes-Picot.

That was the secret 1916 treaty by which the British and French carved up the Ottoman Empire, with the Brits taking Transjordan and Iraq, and the French Syria and Lebanon.

Sykes-Picot stuck in the craw of Osama bin Laden. Now his most fanatical followers have given him a posthumous triumph.

President Obama said over the weekend the Islamic State of Iraq and the Levant, which seeks to create a caliphate out of the Sunni lands of Syria and Iraq it occupies, poses a threat to the United States.

Obama has thus committed 300 special forces to assist Iraq’s defeated and demoralized army, and there is talk of U.S. air and missile strikes and drone attacks on ISIS, in Syria as well as in Iraq.

This action would constitute a new war. Yet the president, who taught constitutional law, says he does not need Congressional authorization.

He is dead wrong. Not only has he no authority to take America into civil wars in Iraq and Syria, he would be insane to do so without the support of his countrymen, as expressed in a vote by Congress.

Obama is about to make a decision fateful for himself and for his country. Does he not realize he is on the edge of an abyss, about to stumble into a tribal and religious war across the Middle East?

The Iraq we left behind three years ago no longer exists.

It has been divided up into Kurdistan, the Sunni region of the north and west and a Shia-dominated Baghdad and south.

To put the Iraq of Sykes-Picot back together would require thousands of troops to recapture and hold Iraq’s border towns and to reimpose Baghdad’s rule over Anbar and the Sunni Triangle.

As the Iraqi army has been routed from this region, recapturing these Sunni lands could require U.S. troops in numbers to rival the surge that enabled Gen. David Petraeus to defeat al-Qaida in Iraq.

Yet the situation in the Sunni region is more hostile today.

The Sunni do not want U.S. troops fighting to force them back under Baghdad’s rule. Some have welcomed ISIS as allies in the fight to be free of a hated Shia-dominated army and regime.

Some Sunni Arab states are expressing bewilderment the United States seems about to start a war on the Sunni regions. Are we really going to send planes to bomb and kill our former allies, with their wives and children as collateral damage?

Among the Shia volunteers on whose side we would be fighting are the Mahdi Army we fought in Operation Iraqi Freedom. Many have blood debts to collect from U.S. soldiers.

Ayatollah Khamenei says while he might welcome the use of U.S. air power against ISIS, he does not want U.S. troops to return to Baghdad or the Shia south. Is the U.S. Air Force going to become the Condor Legion of the Ayatollah Khamenei?

Assume we intervened massively, led the Iraqi army back into the Sunni north and west, and helped it to recapture Mosul and the border posts. How many U.S. troops would we have to leave behind in Iraq to prevent a future Shia regime from losing its Sunni provinces a third time?

The Iraqi army we trained at a cost of $25 billion and left behind in 2011 folded like a house of cards.

How many times must we do this? And if we defeat ISIS, would not these jihadists simply retreat into the Syrian territories they now occupy, as their privileged sanctuary, to come back and fight another day?

Who wants U.S. troops back in Iraq? The American people do not. Congress does not. Tehran does not. The Shia extremists do not. The Sunnis do not. And ISIS does not.

We would be fighting in a war with enemies in all directions.

Yet, is there not a danger terrorists could use the ISIS-dominated region of Iraq and Syria to plot attacks on us?

Surely. But that would be a far greater threat to Turkey, Bashar Assad’s Syria, the Gulf states and Saudi Arabia than to us.

Let them do the fighting this time. After all, it is their backyard, not ours.

And as we saw on 9/11 and at Fort Hood, Muslim fanatics who want to kill Americans do not need safe havens in Tora Bora to plot and prepare. They can do so in Northern Virginia and Delray Beach.

Rand Paul is right. If Barack Obama wants to take us into a new war, with air attacks and drone strikes or with ground troops, he has a constitutional duty to get Congress to authorize that war.

And if Congress does authorize a new war, at least the voters will know whom to be rid of this November.

Patrick J. Buchanan is the author of the new book “The Greatest Comeback: How Richard Nixon Rose From Defeat to Create the New Majority.”

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